The present invention relates to semiconductor devices. In particular, the present invention relates to optical detectors and optical amplifiers.
There are presently available two classes of devices for generating an electrical signal in response to an applied optical excitation, viz., photoconductors and photodiodes. A photoconductor is a device which undergoes a substantial decrease in resistance upon the absorption of light; it requires that an external voltage be applied during operation, and is capable of gains (ratio of electrons passed to photons absorbed) greater than unity. A photodiode contains a potential barrier and generates a voltage upon illumination, so that it is self-powered, but exhibits no gain. Such devices have been described by Richard H. Bube, "Photoconductivity of Solids", John Wiley & Sons, (1967).
In order to synchronously detect a signal due to a dc light source incident on either of these devices, the light is typically "chopped" by a slotted rotating wheel driven by a motor. Such a chopper therefore can convert a continuous magnitude (dc) light signal into a pulsed (ac) signal, which will generate an ac electrical current in a photo-responsive semiconductor device. Such an ac current can be phaselocked with a synchronous detector such as a lock-in amplifier, to avoid the detection of spurious, extraneous signals. However, such choppers tend to be large and clumsy, the motors consume power, they are expensive, and they may generate unwanted noise.
In addition, it is often desired to amplify the magnitude of a detected signal that was created by an ac light originating from an external source and which contains information. In some cases it would be convenient to do this without relying on an external power supply for amplification, such as in a remote location.
The present invention combines in tandem a photodiode and a photoconductive region. The new device is self-powered and can exhibit gains &gt;1, has no rotating mechanical parts, and relies entirely upon optical effects. No external voltage supply is required.